Did you always find music a living entity – in the sense that you were always a composer, never content with performing only what already existed?

Yes, I've done a lot of thinking about this. Although there wasn't actual parental pressure, I was immersed in music from the beginning and it never occurred to me that not everybody thought it was the most important thing in life.

I don't think it was brainwashing – my father was principal double bass of the London Symphony Orchestra for most of my childhood and there were always musicians dropping in. The LSO used to record for the Mercury label at Watford Town Hall which was close by us so some of them would come back at lunchtime. That orchestra was extraordinary at that time, with players like Barry Tuckwell (horn), who was our lodger for a while, Gervase de Peyer [clarinet], Hugh Maguire, Erich Gruenberg (leaders) and, as leader of the seconds, Neville Marriner [not yet a conductor]. So it was part of my life. Apart from the usual enthusiasm for toy trains and model aeroplanes, my favourite objects as a child were 78s. I liked watching the [HMV] dog go round and round! Apparently I could tell the music from the shades and widths of the grooves before I could read the labels!

Did you know what you were listening to? Can you remember what those works were now?

Absolutely. Some that stand out were Stokowski conducting bits of Sleeping Beauty, Lauritz Melchior singing Siegfried's Forging Song, Bruno Walter conducing Song of the Earth with a picture of Mahler on the label, Koussevitzky doing Daphnis and Chloe, Ansermet's Petrushka and so on. But there was also the music that orchestral players in London were just getting to know at the time, the late 1950s and early 1960s: Mahler, Schoenberg, later Stravinsky. We had incomplete bits too, I remember, which still give me an odd perspective today: sides three and four of Elgar conducting Falstaff, sides seven and 16 of Mahler Two with Ormandy, sides four and five of Stokowski's Rite of Spring! Maybe that's the root of why I'm prepared to release pieces as fragments sometimes.

When did you start writing music?

The moment I could read it, which was really as soon as I had piano lessons – with Robin McGee who was later bass player of the London Sinfonietta – he taught me in exchange for lessons from my Dad. If I never made the decision to be a musician, I did make the active choice of being a composer. Once I was reading music, I began to imitate it. I was a terrible piano student but it was clear I was more keen on making up my own stuff. Dad probably thought it would eventually go away at first. He always wanted me to become a conductor. After a while he asked a couple of people what to do, and they said I'd better have some lessons, so off I went to the Watford School of Music where, luckily, my teacher was John Lambert, who had been a student of Nadia Boulanger. I continued with him privately until I was 16 or 17.

Two things happened in your early-mid teens: you met Britten and your First Symphony [now withdrawn] was performed by the LSO, conducted by you. Were these privileges? Traumas?

Dad was working at Aldeburgh at the time. In fact the bass part of Curlew River was written for him. Of the various people he asked for advice about me, Britten was one. He invited me to tea – of course I was terribly shy – and treated me seriously: was I doing counterpoint? Did I plan my pieces carefully? That kind of thing. He was very good at making you feel what you were doing was important, and as if you might be having the same sort of problems he had. I didn't have lessons, just one or two meetings of this sort. But he kept a watchful eye from afar. When I arrived at the Festival Hall for the infamous premiere of my symphony he had sent a telegram, and afterwards he was the first person who offered me a commission, for the 1969 Aldeburgh festival.

Why infamous?

It's a sort of wound that has never really healed, an occurrence I wish would be calmly forgotten and put away. I never seem to be able to get rid of the bloody thing! I do think it's pretty remarkable that a kid of 14 actually wrote such a thing – though it's VERY withdrawn now. It was very good for me that I heard it – these days one would have sent it off to be done in a workshop, quietly, and that would have been the end of it. But with the LSO it became a nine-day wonder – press photographers on the doorstep next morning and all that. My inclination at such times is to flee as a result. It wasn't until I got a fellowship to Tanglewood, where I was greatly encouraged by Gunther Schuller, that I felt in an environment where I was judged for what I actually did, rather than as a result of my background. Dad was chairman of the LSO at the time, as well as principal bass. It probably wasn't the cleverest psychological ploy and I suspect some of my more troublesome personality traits can be traced back to that time. My rather extreme capacity for self-criticism, for a start …

You have always kept parallel lives as conductor and composer, and managed a substantial corpus of published work – you're currently working on Opus 35. Aims? Regrets?

I wish I had written a hell of a lot more than I have done, and I hope I will be able to do a lot more. I'd like to get to 50 pieces, ideally, though I don't know why. I certainly wish I could afford to keep the two roles, conducting and composing, in better balance. One of the defining events of the past decade was that my daughter Sonya's mum, Sue, died [in 2003. He wrote a requiem, Songs for Sue, in 2006]. Then two years later I got quite ill, too, and I became much more aware that time will eventually run out. Family becomes very important at such times. Sonya – whose birthday is the day after mine – is now a mezzo, training in Baltimore with Phyllis Bryn-Julson and John Shirley-Quirk. Earlier this year she sang [Pierre Boulez's] Le Marteau sans maître! What do they say – what goes round comes round?

You have acknowledged a difficulty finishing commissions. Why?

I find it hard psychologically to switch from the conducting persona to being at home alone. It's almost schizophrenic … I like conducting, and working with other people, it's a very good way of stopping yourself being solipsistic, which is a tendency I can't bear. But I'm not a composer who can just sit down and dash something off. I need lots of time to think, and these days I won't let go unless I've really done what I set out to do. I was very pleased Ophelia's Last Dance went down well and has been recorded. There's been a big piece for the Cleveland Orchestra on the back burner for several years, and I'm now working on a piano concerto for Peter Serkin, partly inspired by Kabuki, which I hope to finish very soon.

Do you go along with the current fashion for finding new ways of getting audiences – for new music, as well as mainstream concerts?

Any attempt to get bigger audiences for new music is good. But the popular image of classical music is still so hung up on anachronistic appearances – tenors in white tie and tails, concerts being introduced by ladies in long frocks. I'd get rid of all that. It's a good idea if people wear roughly the same thing, I suppose, but they don't have to cling on to Edwardian nonsense. These days I much prefer listening at home. I get terribly nervous at other people's concerts! But there's also the amazing pleasure now of so many Proms on TV, especially since BBC4 – and that's a bonus for me living far from London.

You live in Snape, close to the home of the Aldeburgh festival and all those associations with Britten and your childhood …

Yes, it's chance really. I was working at the festival and needed a base. I stayed. It's not just because it happens to be within reach of my favourite concert hall [Snape Maltings], or because of the Britten associations. I live here because it's very beautiful and quiet, and the small town atmosphere is not unlike like growing up in the suburbs 50 years ago. But I live alone now and get quite lonely, so I may not stay forever.

Is "new music" in a different state from when you were growing up as a composer? If so, how?

At that time it was obvious who the important composers were. Stravinsky was still alive, Britten, Shostakovich and Copland were in their prime, as were Tippett and Messiaen. A new group were making their mark: Max[well Davies], Sandy Goehr, Birtwistle, Richard Rodney Bennett, Nicholas Maw. There was a clear path ahead, not really in terms of style but in standards and degrees of seriousness. Now, for a young composer, I think it must be terribly hard to work out what your route should be, unless you have an entirely personal and strong vision – which mostly you don't when you're starting out and few people have anyway. Colin Matthews and I run the Composition and Performance course at Snape. It's very encouraging to see how many different approaches there are among young composers. We try not to get in their way, and keep the advice strictly practical.

One great gift you have given, and continue to give, the music world is your enduring support for younger composers, among them George Benjamin, Mark-Anthony Turnage, Julian Anderson.

I've tried to make up for all the ridiculous privileges I've had myself – growing up "in" an orchestra, having my music played publicly at an absurd age – so in a way I'm paying my dues, but I'm happy to. I don't like selfish, isolationist attitudes and I really am interested in what others are doing. It was particularly nice for me to help Minna Keal [1909-1999] get her works performed in her 70s – the opposite end of life from my own start. If you've got an ability you should use it to help your colleagues, anyway. There's some wonderful young talent coming up these days: Helen Grime, Luke Bedford, Ryan Wigglesworth, Charlotte Bray, Sean Shepherd for starters.

What inspires you outside music?

I'm crazy about art – though I have no aptitude for it at all. I'm a fanatical gallery-goer when I'm on tour. I'm nuts about the Flemish Renaissance painter Joachim Patinir, a contemporary of Dürer. He's a large-scale miniaturist! And the Russian illustrator Ivan Bilibin. More recently I've discovered Georges de la Tour, and have come to appreciate Goya hugely, in much the same way that I've come to love Beethoven much more than I did …And yes, you could say it all relates to how I think about music. It's no coincidence that I did my operas with an artist [Maurice Sendak]. I'm thrilled they're being staged at Aldeburgh and the Barbican, for the first time since Glyndebourne, by and for a new generation who didn't see them the first time. And for once I can sit back and watch. That's a wonderful 60th birthday present.

This is a longer version of an article which appeared in the June issue of BBC Music Magazine.